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Selection of process parameters for vacuum heat treatment
The degree of vacuum selection depends on the chemical composition of the parts being processed and the heating and quenching temperature. In the heat treatment quenching temperature range under normal pressure, the alloying elements on the surface of the part generally do not evaporate, but heating under vacuum conditions sometimes causes the evaporation of alloying elements on the surface of the part to be severe, which causes the surface brightness of the part to decrease and the surface structure to change. The mechanical properties are degraded. With the increase of heating temperature and the prolongation of holding time, the higher the degree of vacuum, the more serious the evaporation of alloying elements, especially some alloy steels with higher Cr and Mn elements. However, the degree of vacuum cannot be too low, and the degree of vacuum that is too low does not protect, thereby losing the meaning of vacuum heat treatment. Therefore, the vacuum degree should be reasonably selected during vacuum heat treatment, and the higher the vacuum, the better.
For all metals, the vacuum selection principle of “low temperature, high vacuum, high temperature and low vacuum” should be followed during operation. For carbon steel, heating above 650 °C, under low vacuum, can achieve non-oxidative heating; for high-strength steel 30CrMnSiA and martensitic stainless steel 95Cr18 and other materials, heating at 850 ° C or higher, vacuum should be No oxidation heating can be achieved by selecting 13.3 to 1.33 Pa. Special attention is paid to the fact that some materials, at the same vacuum, tend not to oxidize at high temperatures and to oxidize at moderate temperatures. Such as: 20Cr13, 40Cr13 and other martensitic stainless steel, heating at a vacuum of 6.5Pa, heating at a temperature of about 1050 °C without oxidation, but oxidation at 600 ~ 700 °C. Therefore, when vacuum heating, the degree of vacuum used should be different for different temperature ranges.
In order to ensure the surface roughness of the part without evaporating the alloying elements on the surface of the part, the general correct operation should be to first pump the furnace to a higher vacuum and then charge the high-purity nitrogen to maintain the pressure in the furnace at 26.6. Heating at ~200Pa can obtain better actual production results.